Our responsibility to people in public is to not make things needlessly harder; accept people as you find them unless there’s genuine intervention called for (they’re sobbing, having a medical emergency, that sort of thing). Their expression isn’t your business and no one’s entitled to control that but them. That is courtesy at its most basic and it’s confusing to me how anyone could think the expression on another person’s face somehow costs them something.
How would the frown of another person possibly be a detriment?